Geological volumes (or reservoirs) are defined as 3D domains which contain fluids (such as oil, gas and/or water).
It is specified that such geological volumes often include singular topological surfaces such as faults, horizons and/or the limits of the reservoir itself.
An “horizon” is defined as an interface between two subdomains of a geological volume—these subdomains being typically two layers made of different materials.
These singular surfaces can furthermore be irregular.
The modelling of the behaviour of geological volumes aims in particular at simulating the flow of fluids through and within the geological volume.
Such simulation is carried out by software programs which compute the behaviour of finite volumes (called “cells”), which cells form a partition of the geological volume.
This partition (also called “grid”) represents the geological volume in the “geological domain” (i.e. in the actual physical domain where the geological volume is).
Software programs such as mentioned above are referred to as “flow simulators”.
In order to run a flow simulator, it is therefore necessary to build cells which form a partition of the geological volume.
Each cell of such a partition is associated to some information which has to be memorized in the computer system which runs the flow simulator.
This information typically include for any given cell C0:                The volume of C0,        The permeability tensor of C0,        The list of the cells which are adjacent to C0,        For each cell C1 adjacent to C0, the parameters defining the common face shared by C0 and C1.        
Until recently, two types of known methods have been used for building 3D partitioning (or 3D “grids”) consisting of sets of adjacent cells as mentioned above.
Such 3D grids can be “structured” (as illustrated in FIG. 1) or “non-structured” (as illustrated in FIG. 2).
In both cases, it is necessary to adapt the geometry of the cells in order to run the flow simulator as efficiently as possible.
In particular, the geometry of the cells has to be defined so as to avoid undesirable effects such as having cells intersecting singular surfaces (such as mentioned above) of the geological volume.
Other constraints are associated to the definition of such cells defined in the geological domain: among others, these cells must be aligned with minimal distortion and/or size variation.
Such construction of the cells in the geological domain therefore implies constraints associated to the definition of the geometry and topology of the cells.
These constraints can make the process of building the cells very complex, in particular because the construction of the cells as mentioned above implies coding:                the geometry of the cells (in particular the geometry of the faces of each cell), and        the topology of the cells (in particular information describing the faces of the cells and information allowing the identification of the cells adjacent to any cell).        
This constitutes a drawback of such construction of the cells.
Furthermore, to be honored, these constraints often necessitate to make approximations and/or simplifications on the geometry/topology of the singular surfaces of the geological volume.
Also, they imply that the geometry and topology of the cells has to be memorized, in association with the information mentioned above. This increases the memory space which is required in the computer system used for running the flow simulator.
Moreover, when considering a given point of the geological volume, the process of finding which cell said point is associated to typically necessitate to scan all (or a large number of) cells to find out if the point is contained in the cell. This makes the exploitation of the cell grid burdensome.
Finally, the determination of the permeability tensor of a cell can reveal quite difficult, specially in the case of a non structured grid.
It thus appears that the known methods for building cells are associated to some drawbacks and limitations.
It is to be noted that an advanced approach has been proposed recently for modelling the properties of the cells (but not for building said cells).
This approach has been described in “Space-time mathematical framework for sedimentary geology” (Mathematical Geology, Vol. 36, No 1, 2004).
This advanced approach implies the association of the geological domain with a parametric domain where singular surfaces such as faults and horizons can be managed in a simple manner.
And WO 03/050766 discloses a method for the 3D modelling of a geological volume which presents a variant which can be used in combination with the advanced approach mentioned above.
However, the method disclosed in WO 03/050766 still requires the definition of the cells in the geological domain in order to model the geological volume.
Thus, the method disclosed in WO 03/050766 does not resolve in itself the drawbacks and limitations mentioned above.
An objective of the invention is to avoid these drawbacks and limitations.